Method for manufacturing a single crystal of nitride by epitaxial growth on a substrate preventing growth on the edges of the substrate

ABSTRACT

A method for manufacturing a single crystal of nitride by epitaxial growth on a substrate appropriate for the growth of the crystal. The substrate includes, deposited on the edges of its growth surface, a mask appropriate to prevent growing of the single crystal on the edges of the substrate.

The present patent application is a national phase application ofInternational Application No. PCT/EP2006/069474, filed Dec. 8, 2006.

The present invention concerns a new method for the manufacture ofsingle crystals of nitrides, such as gallium nitride (GaN), aluminiumnitride (AlN) and mixtures thereof, by epitaxial growth on a substrate.

Methods for the epitaxial growth of single crystals of nitrides are wellknown in the art. Properties of single crystals of various sizes werealready disclosed by Chu & al in 1974 (J. Electrochem. Soc.: SOLID-STATESCIENCE AND TECHNOLOGY, January 1974, 159-162). Improved methods fortheir preparations are the object of several patents and patentapplications, including U.S. Pat. Nos. 6,812,053, 6,582,986, 6,803,602,6,955,977, 6,468,347, 6,693,021, 6,413,627, 6,001,748, 6,156,581,6,765,240, 6,972,051, 6,533,874, 6,440,823, 6,156,581, 5,661,074,5,585,648, 6,958,093, 6,765,240, 6,972,051, 6,596,079, 6,447,604,6,596,079, 6,325,850, US 2004/0137732 and WO 2005/031045.

These methods generally comprise growing the crystal on a substrate,said substrate comprising a growth surface. One critical step forobtaining a free standing crystal is to separate the single crystal fromthe growth layer without substantially damaging the crystal. Differentsolutions are known to ease such separation, such as including at leastone sacrificial layer in the substrate, between a base layer and acrystalline upper growth layer. However, under the stringent conditionsfor crystal growth, the single crystal may extend its growth to and overthe edges of the growing surface. Such extensions of growth may raiseadditional problems of separation of the crystal from the substrate.

The present invention provides a solution to this problem.

BRIEF DISCLOSURE OF THE INVENTION

The present invention concerns a method for manufacturing a singlecrystal of nitride by epitaxial growth on a substrate appropriate forthe growth of the said crystal, wherein the substrate comprises,deposited on the edges of its growth surface, a mask appropriate toprevent growing of the said single crystal on the edges of thesubstrate. The mask is preferably constituted with resin material.

The substrate preferably comprises a base layer and an upper crystallinelayer compatible with the growth of the nitride crystal, eventually withat least one sacrificial layer between the base layer and the upperlayer.

The present invention also concerns the substrate used for the methodaccording to the invention and the single crystal of nitride obtained bythe same method.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a perspective view of a substrate (1) comprising amask (2) according to the invention.

FIG. 2 represents a cross view of a substrate (1) comprising a mask (2)according to the invention.

FIG. 3 represents a view from the top of the substrate (1), where themask (2) comprises a plane parallel to a plane (16) on the substrate(1).

Detailed description of the invention below may refer to these figures.Except otherwise specified in the disclosure, such references are forinformation purpose only and characteristics of the invention are alsounderstood to be applicable to other embodiments.

DETAILED DISCLOSURE OF THE INVENTION

In the method according to the invention, the substrate (1) appropriatefor the growth of the crystal comprises, deposited on the edges of itsgrowth surface (11), a mask (2) appropriate to prevent growing of thesaid single crystal over the edges of the substrate

According to the present invention, the “edges of the substrate” meansthe periphery (111) of the surface (11) of the substrate and/or the“vertical” edges (15) of the substrate. “Vertical” is used herein byreference to a substrate which growth surface (11) growth would bemaintained horizontal.

The mask (2) preferably covers at least the periphery (111) of thegrowth surface. Most preferably it covers both the periphery (111) ofthe growth surface (11) and the vertical edges (15). It comprises aperipheral element (21) and a vertical element (22). Both peripheral(21) and vertical (22) element are preferably parts of a same continuouselement.

Substrates (1) are well known in the art of epitaxial growth of crystalssuch as nitride single crystals. Such substrates are disclosed in theart, published, inter alia, in all patent and patent applicationsmentioned above and incorporated herein by reference. They generallycomprise a base layer (13) and an upper crystalline layer (12)compatible with and promoting growth of the single crystal. The baselayer has a first mechanical function to support the growth layer andthe single crystal to be grown. The upper crystalline layer has a mainfunction to allow growth of the crystal. In a first embodiment of theinvention, the base layer and the upper crystalline layer are the same.In other preferred embodiments of the invention, the base layer and theupper crystalline layer are two distinct layers. These two layers may beconstituted with the same material or with two different materials.

The upper crystalline layer is advantageously chosen among the groupconsisting of sapphire, spinel, GaN, AlN, GaAs, Si, SiC (6H—, 4H—, 3C—),LiAlO₂, LiGaO₂, ZrB₂, HfB₂ and mixtures thereof, preferably an AlN orGaN crystalline material.

When the base layer is distinct form the upper crystalline layer, it maybe made of any material compatible with the support and recovery of thesingle crystal after growth. It can be made of amorphous or crystallinematerials. In preferred embodiments, the base layer is made ofcrystalline materials. It can be, like for the upper crystalline layermade of sapphire, spinel, GaN, AlN, GaAs, Si, SiC (6H—, 4H—, 3C—),LiAlO₂, LiGaO₂, ZrB₂, HfB₂ and mixtures thereof. The base layer ispreferably a sapphire layer.

In preferred embodiments of the invention, the substrate comprises atleast one sacrificial layer (14) between the base layer and the upperlayer. Such sacrificial layers are well known in the art, such asdisclosed in WO 2005/031045 incorporated herein by reference. It ispreferably made of silicium.

In preferred embodiments, the resin is selected among the groupconsisting of photosensitive resins, particularly used inphotolithography. Such resins are well known in the art and includesproducts sold under names AZ1512HS or AZ1518HS by Clariant ElectronicsDivision (Rohm & Haas Division), MiPr 6512/6517 by Fujifilm ElectronicMaterials Inc., WPR 1020 by JSR Micro, Inc or SIPR 7120-20 by SHIN ETSUMicroSi, Inc.

The resin allows designing any form to the substrate for the singlecrystal to be grown. These forms may include marks (22), like planes, tokeep information on the crystalline orientation for the single crystalafter growth.

With the crystal having a lateral growth along with a vertical growth,the width W is the width necessary to prevent the crystal to grow overthe edges of the substrate. The minimum width is therefore dependentupon the thickness of the final crystal intended to be grown. Themaximum width W then depends upon the size of the single crystal. Inmost common embodiments, the width W of the mask is at least about 1 mm,comprised from about 1 mm to about 2 mm.

The thickness T of the mask will be dependent upon at least two factorsknown to the skilled person:

-   -   the method used for the application of the mask material onto        the growth surface of the substrate, and    -   the need for the mask to resist the temperature conditions used        for epitaxial growth of the single crystal.

Methods used for application of the mask onto the growth surface of thesubstrate are known for the various mask materials susceptible to beused. As preferred embodiment, the resin may be applied either by“painting” or by printing like ink jet printing. Applying the resin bypainting needs after application to cure the resin at the appropriateshape using usual technology well known in photolithography, like usingfurther masks to avoid exposure of the resin during the curing step tobe able to remove the uncured resin and maintain only the cured resinwith the appropriate shape. On the other hand, using a printingtechnology such as jet ink printing technology may allow applying theresin only where it is needed and then cure it without need of using amask and removing the uncured resin.

In preferred embodiments the mask has a thickness T of from about 1 toabout 3 μm, preferably from 1 to 2 μm.

The shape and size of the growing surface defined by the mask may varyupon the desired shape and size of the crystal to be produced. Accordingto a general embodiment, said surface is circular, eventually truncatedwith planes (22) to keep information on the crystal orientation. Saidsurface generally comprises a circle. The diameter of said circle isgenerally of at least 50 mm.

The single crystal of nitride obtained by the method according to theinvention preferably comprises GaN, AlN and mixture thereof, preferablyGaN. In preferred embodiments, it consists essentially of GaN, AlN andmixture thereof. Nitride crystals according to the invention may berepresented with the following formula:Al_(x)Ga_(y)In_(z)NWherein x+y+z=1 and at least one of x+y+z is not 0.

It may however comprise other elements well known in the art of nitridecrystals, like oxygen. Such doped crystals and the methods for theirpreparations are known in the art. The relative proportions ofadditional elements in these doped nitride crystals may vary accordingto the final properties of the crystal intended to be prepared.

Thickness of the single crystal depends on the time allowed for itsgrowth. Growing conditions and ways to obtain crystals of differentsizes are well known in the art including in the publications citedabove. The single crystal obtained has generally a thickness of morethan 1 μm, preferably from 100 μm to 10 cm, more preferably from 500 μmto 3 mm.

After growth, the single crystal is preferably removed from thesubstrate layer and eventually wafered. Methods for removing the crystalfrom the substrate and methods for their wafering are well known in theart of single crystals production. The top and the bottom surface of thefree single crystal as well as the edges are generally further polishedor grinded using method also known to the skilled person.

The present invention also concerns the substrate as defined above,comprising the mask. It also concerns the single crystal on thesubstrate and the crystal obtained by the method according to theinvention.

EXAMPLE 1 Production of a Free Standing GaN with a Sacrificial LayerUsing a Mask Deposited on the Periphery of the Substrate

The separation process uses a sacrificial layer of Silicium (Si),although other material could be used, which thickness is 0.3 μm. (111)Si is deposited by CVD on (0001) sapphire. Other crystallographicdirections for the Si sacrificial layer like <110> are suitablespecifically for the growth of non polar {11-22} a-plane GaN, thishowever requires R-plane sapphire as starting substrate. Advantageouslythe (111) Si layer is deposited directly in the MOVPE reactor from puresilane.

Direct growth of GaN on (111)Si by MOVPE does not lead to high qualitylayers. Conversely, a layer like SiC or AlN has proven to be beneficialfor the quality of the GaN layer. Thus 0.15 μm thick AlN layer isdeposited on (111)Si by MOVPE using growth process well known by theskilled person in the art.

At this stage, the substrate (1) is formed by a base layer (13) made ofsapphire, a sacrificial layer (13) made of silicon and a crystallinelayer (12) compatible with GaN growth made of AlN.

According to the present invention the next step is to form a mask atthe periphery of the substrate. To this end, the substrate is loaded ina resist dispensing set up; the resist for example Rohm and Haas S1813is poured and spin to form a thin film on the substrate. After resistcuring on a hot plate, UV lithography is used to transfer a pattern froma lithography mask to the resist film i.e. a ring like pattern coveringboth the vertical edge and 1.3 mm width from the outer diameter. A flatis included in the pattern to be aligned parallel to the crystallineorientation flat of the sapphire base. A final step consists in solvingthe resist in a specific developer to expose the growth area 11 forfurther regrowth by HVPE technique.

The HVPE growth starts at about 950° C. In this low temperature step ofgrowing a thick GaN layer by HVPE, the temperature is set at 930-950°C., the partial pressure of HCl, pHCl at 0.03, of NH3, pNH3 at 0.24, thecarrier gas, H2, pH2 at 0.73 respectively. The growing pressure was 2.6kPa. Growth is carried out during 10 hours at a growth rate of about 100μm/h.

At this rather low temperature, GaN grows in the form of coherentisland, randomly distributed with random size, having facets.

After the low temperature step, growth temperature is set up to1015-1020° C., with the following values for the partial pressures ofactive and carrier gas:

pHCl, 0.02

PNH3, 0.31

pH2, 0.67

Working pressure: 2.6 kPa

In these growth conditions, the lateral growth rate is enhanced and thelayer becomes flat. This corresponds to 2D growth of GaN parallel to thebasal plane.

During this second high temperature step, where lateral growth isenhanced, the sacrificial Si interlayer evaporates and a cleanseparation takes place between the base material surface and thetypically 1 mm thick GaN layer grown by HVPE.

Due to the existence of the mask following the present invention, no GaNgrowth occurs or a very thin and brittle polycrystalline GaN deposit isformed on the periphery of the base material. Therefore the edges of theGaN thick layers are bonded neither to the periphery 111 of the baselayer 14 nor the vertical edge of the base layer 14. This mask isextremely efficient to avoid the building of detrimental strain into thethick GaN layer from its edges. Such a compressive or tensile strainwould build up during cooling down after the growth due to the possiblethermal coefficient mismatch between GaN and the base material; whenexisting this strain would initiate cracks in the thick GaN layer.

Another advantage of the present invention is the very easy andreproducible separation of the thick GaN layer from the base materialsince the GaN layers is not bonded to the surface 11 due to thesacrificial layer disappearance and since no bonds exists at its edges.No mechanicals efforts needs to be applied that may damage or break theGaN layer.

Such a process produces free standing GaN 2″ wafers with TDs density inthe low 10⁶ cm⁻².

What is claimed is:
 1. A substrate appropriate for the growth of a nitride single crystal by epitaxial growth, wherein the substrate comprises a base layer and an upper crystalline layer compatible with the growth of the nitride crystal and sacrificial layer between the base layer and the upper layer, and wherein the substrate comprises a mask deposited on an edge of a growth surface configured to prevent growing of the nitride single crystal on the a vertical surface of the substrate.
 2. The substrate of claim 1, wherein the mask comprises a resin.
 3. The substrate of claim 1, wherein the mask has a width W of about 1 mm.
 4. The substrate of claim 1, wherein the mask has a thickness T of from about 1 to about 3 μm.
 5. The substrate of claim 1, wherein the upper crystalline layer comprises a material selected from the group consisting of sapphire, spinel, GaN, AlN, GaAs, Si, SiC (6H—, 4H—, 3C—), LiAIO₂, LiGaO₂, ZrB₂, HfB₂.
 6. The substrate of claim 1, wherein the upper layer comprises AlN.
 7. The substrate of one of claims 1, wherein the base layer comprises sapphire.
 8. The substrate of claim 1, wherein the sacrificial layer is in direct contact with the base layer and the upper layer.
 9. The substrate of claim 8, wherein the sacrificial layer comprises silicon.
 10. The substrate of claim 1, wherein the the nitride single crystal is configured to be grown in an area within the perimeter of the mask.
 11. The substrate of claim 10, wherein the mask comprise a circle having a diameter of at least 50 mm.
 12. The substrate of claim 1, wherein the upper layer comprises GaN.
 13. The substrate of claim 1, wherein the nitride single crystal comprises a thickness of more than 100 μm.
 14. The substrate of claim 1, wherein the mask has a ring-like shape comprising a flat configured to align with an orientation flat of the substrate.
 15. The substrate of claim 1, wherein the mask comprises a vertical element overlying the vertical surface of the substrate.
 16. The substrate of claim 15, wherein the vertical element is continuous with a peripheral element of the mask, wherein the peripheral element overlies a periphery of the growth surface of the substrate. 